In its fifth season (1991-1992), TNG averaged 11.5 million viewers. During that same year, Law & Order -- a major network show that was successful enough to run 20 seasons -- averaged 12.3 million. The Cosby Show averaged 13.6 million.
Regardless of where or at what time it was airing, TNG was pulling viewership numbers comparable to the major network shows of the time. And although airing in syndication has some advantages in terms of competition and such, it also has some disadvantages -- such as getting viewership in markets that choose to air the show at 3 am on Thursday.
My point is not that TNG's situation is directly comparable to a network show. But it WAS pulling network-level ratings, and I think the amount of popularity it enjoyed at the time could have translated to a network without difficulty. And I think it's the only Trek show that really could have accomplished network success at that level.